Sheep erythrocyte spectrin and rabbit skeletal muscle actin have been purified and their interactions studied by viscometry and ultracentrifugation. It has been found that the tetrameric form of spectrin crosslinks F-actin filaments to form a gel and that spectrin dimer binds to, but does not crosslink, F-actin. Neither phosphorylation of the spectrin, nor dephosphorylation, was found to affect its interaction with actin. A high molecular weight oligomeric complex of spectrin, actin, and several minor proteins, have been isolated from sheep erythrocyte ghosts. We have found that this complex can induce the polymerization of G-actin. Studies on the mechanism of action of this complex are in progress.